The present invention relates to a shock absorber, and more particularly to a shock absorber which may find its application in controlling closing and opening movement of a closure of a box, such as a glove box of an automotive vehicle.
The shock absorber of this kind has been proposed. Japanese Utility Model application laid-open publication No. 59-27558 discloses a shock absorber comprising a cylinder having a cylindrical bore, a peripheral wall defining the cylindrical bore, and an end wall closing one axial end of the bore. An actuator piston has a piston head slidably disposed in the bore and a rod extending out of the cylinder through the opposite open axial end thereof. The piston head cooperates with the end wall to define a chamber. The end wall is formed with a tapped air hole with which an adjustment screw is engaged for providing a fluid flow passage between the chamber and the ambient atmosphere. The piston head has a valve element formed of an elastomer fixedly mounted thereto. The valve element has a tubular base portion fit in a circumferential groove formed around the piston head, and an integral cup-shaped sealing lip portion. The cup-shaped sealing lip portion is compressed radially within the bore, thus keeping an engagement with the peripheral wall to seal a clearence around the piston head. The cup-shaped sealing lip portion is flexed to reduce the engagement with the peripheral wall during movement of the actuator piston relative to the cylinder in a direction to contract the shock absorber even though it comes into firm engagement with the peripheral wall during an opposite movement of the actuator piston in a direction to extend the shock absorber. Thus, although a smooth contracting movement of the actuator piston is always ensured, the extending movement of the actuator piston is effectively resisted to provide a slow movement. If the shock absorber stays in its fully contracted position for a long time, the valve element tends to stick with the peripheral wall, making the movement of the actuator piston difficult, rendering the shock absorber inoperative. To cope with this problem, a plurality of projections are formed on the peripheral wall within an area where the valve element will be situated when the actuator piston is received deeply into the cylinder, so that the valve element rides on the projections and thus is separated from the peripheral wall.
According to this known shock absorber, the rate of air flowing through the air hole remains constant even though it is manually adjustable by turning the adjustment screw, so that although it extends at a desired slow speed if it is pulled out of the cylinder with a small force, the actuator piston no longer extends at such desired slow speed if it is pulled out of the cylinder with a large force. Thus, if it is used as a shock absorber for a glove box of an automotive vehicle, for example, the glove box will be opened at a high speed until it abruptly brought to a halt at its fully open position if a load carried by the glove box is heavy. Since, the rate of air flow through the air hole has to be adjusted to a value large enough to provide smooth contracting movement, it is the valve element which plays a main role in generating a sliding friction during the extending movement thereof.
Therefore, this known shock absorber does not meet a demand that the actuator piston should extend at substantially the same slow speed regardless of the magnitude of a load applied. Besides, since it has a special contour, the valve element is difficult to mount and has a short operating life.
Japanese patent application laid-open publication No. 58-174038 discloses a shock absorber that is designed to be used for a glove box of an automotive vehicle. This known shock absorber comprises a cylinder having a cylindrical bore, a peripheral wall and a closed end wall formed with an air hole, an actuator piston including a piston head slidably disposed is the bore and a rod extending out of the cylinder through an open end of the cylinder. The piston head cooperates with the closed end wall to define a chamber that is allowed to communicate with the ambient atomosphere through the air hole. The peripheral wall is formed with air holes opening into the chamber. A rubber tube wrapping around the peripheral wall to cover the air holes. During extending movement of the actuator piston relative to the cylinder when the associated glove box is being moved from its closed position to fully opened position, a vacuum develops in the chamber and air is allowed to enter the chamber only through the air hole formed through the end wall because the air holes formed through the peripheral wall are closed by the rubber tube. The air flow through the air hole formed through the end wall is restricted by a throttle valve or a ring-shaped one-way type throttle valve formed of a rubber. As a result, the actuator piston extends out of the cylinder at a desired slow speed. During contracting movement of the actuator piston when the glove box is closed, a positive pressure develops in the chamber to push open the air holes formed through the peripheral wall by lifting the rubber tube. Thus, air can escape from the chamber not only through the end air hole, but also through the peripheral end holes.
This, known device suffers from the same problem as the first mentioned known device because the rate of flow of air between the ambient atmosphere and the chamber during extentding movement of the actuator piston is not variable and constant regardless of the magnitude of a load applied.
The present invention aims at improving a shock absorber of the type wherein a speed of movement of an actuator piston is largely dependent on a rate of fluid flow into and out of a chamber defined by a piston head of the actuator piston such that extending movement of the shock absorber is kept at substantially the same slow speed regardless of the magnitude of a load applied.